The present invention relates to a method of producing mold parts in accordance with the cold box process, as well as to a mold part and a molding tool.
Mold parts of synthetic resin bound silica hereinbelow referred to as casting molds, include the mold parts having cores inserted therein and are an important basis for the mass production of high quality castings. Various manufacturing processes differ from one another in the type of synthetic resin used and their catalytic curing. The catalysis is performed either by heating or at room temperature by adding a catalyst. Heat curing manufacturing processes are known under the names of hot box, warm box and thermoshock processes. These processes, however, have been increasingly replaced by cold curing processes since they provide for the advantage in energy saving and improved working conditions. In addition, the manufacture of mold parts can be carried out in plastic mold tools.
It is known that the molded parts produced in accordance with various processes and also in accordance with the cold box process are provided after the formation of the mold parts with a layer on their sides which forms a mold cavity. The application and drying a layer require additional working operations and also a waiting time till casting of the casting mold and thereby sufficient time for drying the layer.
In the area of cold curing, the so-called cold box method has achieved a high degree of importance worldwide. Very high production outputs are obtained on automatic production lines. This method uses polyurethane as a binder. The starting components for use now are isocyanate and a phenolic resin, however, other binder combinations are also possible. They are mixed with silica sand in ratio of approximately 1-2 parts by weight. The thus produced molding material is introduced into the mold tool in automatic production of casting molds and immediately after this is cured in a cold tool by passing a catalyst gas, usually dimethylethylamine.
From technical and economical reasons and especially for reducing the environmental impact, the use of lowest possible binder content in the casting practice is desirable, which, however, leads to serious weaknesses of the cold box method.
Cold box binders contain approximately 30-40% of various solvents which are required for low viscosity, high reactivity of the binder, good blowing property of the mold material mixture and adequate strength. These high solvent quantities lead to considerable environmental impacts during processing and pouring off. Lower contents than those mentioned above impair however the strength of the mold parts, especially the mold part surfaces. The edge strength is affected and the mold parts become in their entirety sandy and brittle. Thereby the cold box method loses its usability. Polyurethane-bound mold parts with sufficient solvent and binder contents have good strength immediately after their manufacture. They are however very moisture sensitive and lose their strength within a short time in condition of high air moisture. High air moisture is however, unavoidable in casting. In addition, cold box cores are often treated with water slurry and introduced into wet casting molds thereby are subject to severe moisture damage. This especially impairs the mold part quality since this damage progresses from outside inwardly and thereby affects the especially important mold part surfaces. A highly undesirable strength gradient is produced with low outer strength and high inner strength.
This strength gradient is a disadvantage for a further reason since it makes difficult core destruction after pouring off. Poor core destruction of cold box mold parts is especially dreaded in light metal alloys casting. Remnants of cores are often very difficult to remove from the cold casting and require high fettling costs and extreme working conditions in the fettling shop.
The casting practice made an attempt to counteract the difficulties caused by low surface strength, by increasing binder contents and introduction of coating materials. High binder contents however additionally impair the core destruction since the strength in the interior of the core is increased very high. In addition, moisture damage takes place in the interior of the core to a smaller degree than in the surface layers. Cold box mold parts thus have the serious disadvantage of an undesirable strength distribution. Also their strength increases within 24 hours after the fabrication.